The box and whiskers plots represents the historical distributions of seasonal temperature and precipitation associated with 3 different ENSO categories - El Nino, La Nina, and Neutral events for each climate division. The 3-month categorizations of ENSO events that were used are available at :
ENSO events

The 3-month temperature or precipitation data for a specific ENSO category is fitted to a distribution (Gaussian for temperature, Gamma for precipitation). Then a curve is fitted to the resulting distribution. From this curve, various percentiles are obtained and are plotted in the box and whiskers plot (See Fig 1.) .

How To Interpret The Plot Fig. 1

The box and whiskers plot is a way to show the range of percentiles that the temperature and precipitation fall in for the 3 different ENSO categories.

A large spread in the 3-month temperature/precipitation box and whiskers shapes between the 3 ENSO categories would indicate that ENSO has a larger influence on the temperature/precipitation for the specified climate division.

For example : if all 3 box and whiskers plots for the 3 ENSO categories have similar temperature values, this would indicate that the temperature has typically been the same regardless of the ENSO event category. If the temperature plot for the El Nino events are greatly higher than for La Nina or neutral, this would indicate that in the past the temperature has been significantly greater during El Nino events.

Dates used for data

The data used to make these plots are from 1950-present. The specific years of episodes for each of the seasons' plots can be found by going to the ENSO composites page :

You can click on the corresponding 3-month period on the left bar. Under the Composite map graphics are a list of years used for these cases. These are the same used for these ENSO Box & Whiskers Plots.